The electromagnetic spectrum band of between about 0.3 THz to 10 THz is often referred to as the “terahertz (THz) gap” due to the difficulties in efficiently generating, manipulating and detecting THz radiation. THz electromagnetic waves may be applied to a diverse range of applications, ranging from non-destructive imaging, spectroscopic sensing, to ultra-high bit rate wireless communication. An optical modulator is a key component widely used for beam manipulation, imaging, optical communication, as well as active mode locking and others. However, in the THz spectrum, the development of fast and efficient modulators is lagging behind.
Previous attempts to build a THz modulator include the use of a two-dimensional electron gas (2 DEG) in semiconductor structures whose electrons can be injected or depleted by an applied gate voltage. However, these solutions can only modulate the THz wave amplitude by a few percent. By incorporating metamaterials or plasmonic structures to enhance the interaction between the incoming light and the 2 DEG, the modulation depth may be increased to 30%. Ultimately, the modulation depth of these devices may be limited by the achievable tunability in electron gas density (up to ˜1×1012 cm−2 for 2 DEG or bulk semiconductor). Other solutions may include electrically gating a large-area graphene sheet, which may achieve a modulation depth of about 15%. By taking advantage of the electric field enhancement effect in an optical cavity, plasmonic structure or metamaterials, modulation depth of 64% may be achieved in graphene-based THz modulators. However, further improvement remains difficult. In addition, the available graphene modulators were studied as isolated components, of which a large active area of several millimeters by several millimeters may be typical to facilitate optical arrangement. The consequent large time constant of the effective resistor-capacitor (RC) circuit of the device package may limit the modulation speed of these graphene modulators to only about 13 MHz, at the same level of those of their semiconductor counterparts, despite the ultra-high intrinsic carrier mobility of graphene (>20,000 cm2V−1 s−1 for graphene on SiO2).
Therefore, there is a need for an improved graphene modulator that may be able to achieve a better modulation depth and a faster modulation speed, as compared to existing modulators.